At least 57 prisoners were killed, including 16 who were decapitated, during a prison riot in Brazil's northern Para state on Monday, authorities said.

The riot broke out at the Altamira Regional Recovery Center at around 7 a.m. local time (1000 GMT) and ended around noon, according to the state's Superintendency of the Penitentiary System (Susipe).

Members of the Comando Classe A gang set fire to a cell where rival gang members from Comando Vermelho were kept, Susipe said in a statement.

"It was a targeted attack," Para prison official Jarbas Vasconcelos said. "The aim was to show that it was a settling of accounts between the two groups, not a protest or rebellion against the prison system."

According to Susipe, the prison was built to hold 163 inmates, but was housing 343, more than twice its capacity.

It was the worst prison killing registered this year in Brazil, surpassing a deadly riot in neighboring state Amazonas, where 55 inmates were killed in May.

According to World Prison Brief, Brazil has the world's third-largest prison population of 726,354 inmates as of June 2017. The population is roughly double the capacity of the nation's jails.

Xinhua